You can use average model (AM) to formulate the relation between voltage and current of each H-bridges. This method models the voltage across of H-bridges in terms of capacitors voltage and switching function. Also, the current of each phase is proportion to capacitor's current and switching function. Furthermore, the voltage and current of load is proportional to capacitors voltage and current of each phase. So, you can easily derive the relation between active and reactive power components of load and voltage and currents of capacitors. As you now, power factor is the ratio of active power to apparent power, so you can reach to your solution.